Three Reaction Pathways in the H + HCO → H2 + CO Reaction

Roaming pathways and survival probability in real-time collisional dynamics of cold and controlled bialkali molecules

Perfectly controlled molecules are at the forefront of the quest to explore chemical reactivity at ultra low temperatures. Here, we investigate for the first time the formation of the long-lived intermediates in the time-dependent scattering of cold bialkali \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{23}\hbox {Na}^{87}$$\end{document}23Na87Rb molecules with and without the presence of infrared trapping light. During the nearly 50 nanoseconds mean collision time of the intermediate complex, we observe unconventional roaming when for a few tens of picoseconds either NaRb or \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {Na}_2$$\end{document}Na2 and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {Rb}_2$$\end{document}Rb2 molecules with large relative separation are formed before returning to the four-atom complex. We also determine the likelihood of molecular loss when the trapping laser is present during the collision. We find that at a wavelength of 1064 nm the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {Na}_2\hbox {Rb}_2$$\end{document}Na2Rb2 complex is quickly destroyed and thus that the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{23}\hbox {Na}^{87}$$\end{document}23Na87Rb molecules are rapidly lost.

Double-Roaming Dynamics in the H + C2H2 → H2 + C2H Reaction: Acetylene-Facilitated Roaming and Vinylidene-Facilitated Roaming

We report two novel roaming pathways for the H + C₂H₂ → H₂ + C₂H reaction by performing extensive quasiclassical trajectory calculations on a new, global, high-level machine learning-based potential energy surface. One corresponds to the acetylene-facilitated roaming pathway, where the H atom turns back from the acetylene + H channel and abstracts another H atom from acetylene. The other is the vinylidene-facilitated roaming, where the H atom turns back from the vinylidene + H channel and abstracts another H from vinylidene. The "double-roaming" pathways account for roughly 95% of the total cross section of the H₂ + C₂H products at the collision energy of 70 kcal/mol. These computational results give valuable insights into the significance of the two isomers (acetylene and vinylidene) in chemical reaction dynamics and also the experimental search for roaming dynamics in this bimolecular reaction.

Advances and New Challenges to Bimolecular Reaction Dynamics Theory

Dynamics of bimolecular reactions in the gas phase are of foundational importance in combustion, atmospheric chemistry, interstellar chemistry, and plasma chemistry. These collision-induced chemical transformations are a sensitive probe of the underlying potential energy surface(s). Despite tremendous progress in past decades, our understanding is still not complete. In this Perspective, we survey the recent advances in theoretical characterization of bimolecular reaction dynamics, stimulated by new experimental observations, and identify key new challenges.

Roaming Reactions and Dynamics in the van der Waals Region

Roaming reactions were first clearly identified in photodissociation of formaldehyde 15 years ago, and roaming dynamics are now recognized as a universal aspect of chemical reactivity. These reactions typically involve frustrated near-dissociation of a quasibound system to radical fragments, followed by reorientation at long range and intramolecular abstraction. The consequences can be unexpected formation of molecular products, depletion of the radical pool in chemical systems, and formation of products with unusual internal state distributions. In this review, I examine some current aspects of roaming reactions with an emphasis on experimental results, focusing on possible quantum effects in roaming and roaming dynamics in bimolecular systems. These considerations lead to a more inclusive definition of roaming reactions as those for which key dynamics take place at long range.

Collision-induced and complex-mediated roaming dynamics in the H + C2H4 → H2 + C2H3 reaction

Roaming is a novel mechanism in reaction dynamics. It describes an unusual pathway, which can be quite different from the conventional minimum-energy path, leading to products. While roaming has been reported or suggested in a number of unimolecular reactions, it has been rarely reported for bimolecular reactions. Here, we report a high-level computational study of roaming dynamics in the important bimolecular combustion reaction H + C₂H₄ → H₂ + C₂H₃, using a new, high-level machine learning-based potential energy surface. In addition to the complex-mediated roaming mechanism, a non-complex forming roaming mechanism is found. It can be described as a direct inelastic collision where the departing H atom roams and then abstracts an H atom. We denoted this as “collision-induced” roaming. These two roaming mechanisms have different angular distributions; however, both produce highly internally excited C₂H₃. The roaming pathway leads to remarkably different dynamics as compared with the direct abstraction pathway. A clear signature of the roaming mechanism is highly internally excited C₂H₃, which could be observed experimentally.

Collision-induced and complex-mediated roaming mechanisms are revealed for an important bimolecular reaction in combustion.

Liberation of H2 from (o-C6H4Me)3P-H(+) + (-)H-B(p-C6F4H)3 ion-pair: A transition-state in the minimum energy path versus the transient species in Born-Oppenheimer molecular dynamics

Using Born-Oppenheimer molecular dynamics (BOMD) with density functional theory, transition-state (TS) calculations, and the quantitative energy decomposition analysis (EDA), we examined the mechanism of H₂-liberation from LB-H⁽⁺⁾ + ^(-)H-LA ion-pair, 1, in which the Lewis base (LB) is (o-C₆H₄Me)₃P and the Lewis acid (LA) is B(p-C₆F₄H)₃. BOMD simulations indicate that the path of H₂ liberation from the ion-pair 1 goes via the short-lived transient species, LB⋯H₂⋯LA, which are structurally reminiscent of the TS-structure in the minimum-energy-path describing the reversible reaction between H₂ and (o-C₆H₄Me)₃P/B(p-C₆F₄H)₃ frustrated Lewis pair (FLP). With electronic structure calculations performed on graphics processing units, our BOMD data-set covers more than 1 ns of evolution of the ion-pair 1 at temperature T ≈ 400 K. BOMD simulations produced H₂-recombination events with various durations of H₂ remaining fully recombined as a molecule within a LB/LA attractive "pocket"-from very short vibrational-time scale to time scales in the range of a few hundred femtoseconds. With the help of perturbational approach to trajectory-propagation over a saddle-area, we directly examined dynamics of H₂-liberation. Using EDA, we elucidated interactions between the cationic and anionic fragments in the ion-pair 1 and between the molecular fragments in the TS-structure. We have also considered a model that qualitatively takes into account the potential energy characteristics of H-H recombination and H₂-release plus inertia of molecular motion of the (o-C₆H₄Me)₃P/B(p-C₆F₄H)₃ FLP.

Theories and simulations of roaming

The phenomenon of roaming in chemical reactions has now become both commonly observed in experiment and extensively supported by theory and simulations. Roaming occurs in highly-excited molecules when the trajectories of atomic motion often bypass the minimum energy pathway and produce reaction in unexpected ways from unlikely geometries. The prototypical example is the unimolecular dissociation of formaldehyde (H₂CO), in which the "normal" reaction proceeds through a tight transition state to yield H₂ + CO but for which a high fraction of dissociations take place via a "roaming" mechanism in which one H atom moves far from the HCO, almost to dissociation, and then returns to abstract the second H atom. We review below the theories and simulations that have recently been developed to address and understand this new reaction phenomenon.

Toward Understanding the Roaming Mechanism in H + MgH → Mg + HH Reaction

The roaming mechanism in the reaction H + MgH →Mg + HH is investigated by classical and quantum dynamics employing an accurate ab initio three-dimensional ground electronic state potential energy surface. The reaction dynamics are explored by running trajectories initialized on a four-dimensional dividing surface anchored on three-dimensional normally hyperbolic invariant manifold associated with a family of unstable orbiting periodic orbits in the entrance channel of the reaction (H + MgH). By locating periodic orbits localized in the HMgH well or involving H orbiting around the MgH diatom, and following their continuation with the total energy, regions in phase space where reactive or nonreactive trajectories may be trapped are found. In this way roaming reaction pathways are deduced in phase space. Patterns similar to periodic orbits projected into configuration space are found for the quantum bound and resonance eigenstates. Roaming is attributed to the capture of the trajectories in the neighborhood of certain periodic orbits. The complex forming trajectories in the HMgH well can either return to the radical channel or "roam" to the MgHH minimum from where the molecule may react.

Insight into photofragment vector correlation by a multi-center impulsive model

A multi-center impulsive model has been recently developed to characterize the dynamic feature of fragment vector correlation in photodissociation of formaldehyde, H₂CO → CO + H₂, via both transition state and roaming pathways.

Dynamics of chlorine atom reactions with hydrocarbons: insights from imaging the radical product in crossed beams

We present a comprehensive overview of our ongoing studies applying dc slice imaging in crossed molecular beams to probe the dynamics of chlorine atom reactions with polyatomic hydrocarbons. Our approach consists in measuring the full velocity-flux contour maps of the radical products using vacuum ultraviolet "soft" photoionization at 157 nm. Our overall goal is to extend the range of chemical dynamics investigations from simple triatomic or tetraatomic molecules to systematic investigations of a sequence of isomers or a homologous series of reactants of intermediate size. These experimental investigations are augmented by high-level ab initio calculations which, taken together, reveal trends in product energy and angular momentum partitioning and offer deep insight into the reaction mechanisms as a function of structure, bonding patterns, and kinematics. We explore these issues in alkanes, for which only direct reactive encounters are found, and in unsaturated hydrocarbons, for which an addition-elimination mechanism competes with direct abstraction. The results for alkene addition-elimination in particular suggest a new view of these reactions: The only pathway to HCl elimination is accessed by means of roaming excursions of the Cl atom from the strongly bound adduct.

Uncovering the role of intra- and intermolecular motion in frustrated Lewis acid/base chemistry: ab initio molecular dynamics study of CO2 binding by phosphorus/boron frustrated Lewis pair [tBu3P/B(C6F5)3]

The role of the intra- and intermolecular motion, i.e., molecular vibrations and the relative motion of reactants, remains largely unexplored in the frustrated Lewis acid/base chemistry. Here, we address the issue with the ab initio molecular dynamics (AIMD) study of CO2 binding by a Lewis acid (LA) and a Lewis base (LB), i.e., tBu3P + CO2 + B(C6F5)3 → tBu3P-C(O)O-B(C6F5)3 ([1]). Reasonably large ensemble of AIMD trajectories propagated at 300 K from structures in the saddle region as well as trajectories propagated directly from the reactants region revealed an effect arising from significant recrossing of the saddle area. The effect is that transient complexes composed of weakly interacting reactants nearly cease to progress along the segment of the minimum energy pathway (MEP) at the saddle region for a (subpicosecond) period of time during which the dominant factor is the light-to-heavy type of relative motion of the vibrating reactants, i.e., the "bouncing"-like movement of CO2 with respect to much heavier phosphine and borane as main contributor to the mode that is perpendicular to the MEP-direction. In terms of how P···C and B···O distances change with time, the roaming-like patterns of typical AIMD trajectories, reactive and nonreactive alike, extend far beyond the saddle region. In addition to the dynamical portrayal of [1], we provide the energy-landscape perspective that takes into account the hierarchy of time scales. The verifiable implication of the effect found here is that the isotopically substituted (heavier) LB/LA "pair" should be less reactive that the "normal" and thus lighter counterpart.

Quasiclassical trajectory studies of 18O(3P) + NO2 isotope exchange and reaction to O2 + NO on D0 and D1 potentials

We report quasiclassical trajectory calculations for the bimolecular reaction (18)O((3)P) + NO2 on the recent potential energy surfaces of the ground (D0) and first excited (D1) states of NO3 [B. Fu, J. M. Bowman, H. Xiao, S. Maeda, and K. Morokuma, J. Chem. Theory. Comput. 9, 893 (2013)]. The branching ratio of isotope exchange versus O2 + NO formation, as well as the product angular distributions and energy and rovibrational state distributions are presented. The calculations are done at the collision energy of relevance to recent crossed beam experiments [K. A. Mar, A. L. Van Wyngarden, C.-W. Liang, Y. T. Lee, J. J. Lin, and K. A. Boering, J. Chem. Phys. 137, 044302 (2012)]. Very good agreement is achieved between the current calculations and these experiments for the branching ratio and final translational energy and angular distributions of isotope exchange products (16)O((3)P) + NO2 and O2 + NO formation products. The reactant (18)O atom results in (18)O(16)O but not N(18)O for the O2 + NO formation product channel, consistent with the experiment. In addition, the detailed vibrational and rotational state information of diatomic molecules calculated currently for the (34)O2 + NO formation channel on D0 and D1 states are in qualitative agreement with the previous experimental and theoretical results of the photodissociation of NO3 and are consistent with older thermal bimolecular kinetics measurements.

Observation of NH X(3)Σ(-) as a Primary Product of Methylamine Photodissociation: Evidence of Roaming-Mediated Intersystem Crossing?

3+1 Resonance-enhanced multiphoton ionization and photofragment excitation spectroscopy have been used to identify NH X(3)Σ(-) as a primary product of methylamine photodissociation after state-specific excitation to the S1 state. On the basis of standard thermochemical data, NH X(3)Σ(-) can be formed only in conjunction with closed-shell CH4 coproducts, indicating that dissociation must occur on the T1 surface. It is proposed that the mechanism for the formation of triplet NH and CH4 involves intramolecular abstraction between frustrated radical products and is an example of roaming-mediated intersystem crossing.